For understanding angular distributions, spin effects and for access
to the parton distributions (PDFs and TMDs), improving on the existing tools in producing analytical
expressions as simply and economically as possible is beneficial.
Besides the importance of analytical methods for phenomenological study of QCD processes
in the fixed target regime (DY, SIDIS, etc.), handy analytical expressions are also important
for performing resummation of large logarithms, verifying of different identities (like Lam-Tung
relation) involving hadronic structure functions, behavior of the hadronic amplitudes at different
kinematical limits and their expansions in small parameters (like small transverse momentum of
photon, etc.). See, e.g., discussion in Refs. [1]}, [2]}.
